Approximately 40,000 Americans continue to contract HIV disease each year. To reduce HIV rates below this plateau, HIV prevention interventions must be better-focused and targeted toward community population subsets that remain, at greatest risk. Because risk behavior, STDs, and HIV are often concentrated within high-risk social networks, HIV sexual risk reduction approaches directed toward risky social networks are especially needed. Engaging social leaders within these high-risk networks to advise, counsel and communicate HIV prevention messages to their network members is a promising but understudied approach. This research will test an intervention that targets high sexual-risk social networks of two populations, young men who have sex with men (YMSM) and young high-risk heterosexual adult men and women (YHAs). Following a period of in-depth ethnographic formative research, 112 networks will be recruited to participate in the main intervention trial. Socially-active "indexes" observed in high-risk community venues will serve as network entry points. The entire close social network of each index will be enrolled in the study to form a cohort that is longitudinally followed. At baseline, members of all networks will complete A-CASI risk assessment interviews, provide specimens for STD testing, receive risk reduction counseling and STD treatment, and complete sociometric measures to determine network social influence leadership. YMSM and YHA networks will be randomized in equal numbers to intervention and comparison conditions. The sociometrically-identified leaders of intervention condition networks will participate in a group program of 5 weekly sessions plus 4 boosters at fading intervals. The intervention trains and guides leaders in delivering to their personal network members theory-based and culturally-tailored risk reduction messages that target knowledge, norms, attitudes, intentions, and self-efficacy. At 3- and 15-month follow-up points, members of all study networks will be re-assessed on behavioral, biological, and intervention exposure measures. Reductions in unprotected intercourse, increased condom use, changes on risk-related psychosocial scales, increased condom redemption, and lower STD incidence are hypothesized for intervention condition networks. This research, if successful, will provide a model that can be used to focus HIV prevention efforts on community population segments in greatest need.